Understanding the influence of immobilization is key to advancing efforts in green chemistry based on supported catalyst materials. The binding of a model reactant 4-nitroacetophenone with amino and ureido organocatalytic monolayers has been investigated at the acetonitrile/solid interface using a combination of second harmonic generation (SHG) and sum frequency generation (SFG). By changing the ketone concentration in the bulk solvent, binding isotherms at each interface were determined from SHG measurements. Langmuir fitting of these isotherms yielded binding energies consistent with hydrogen bond formation. Surprisingly, the ketone had a lower binding affinity for ureido monolayers compared with its binding behavior at amino-modified surfaces despite the fact that urea can form two hydrogen bonds with carbonyl groups. This lower binding affinity was attributed to the presence of a hydrogen bond network within the ureido monolayer that must be disrupted to facilitate ketone binding. Vibrational SFG measurements of the urea groups in the N-H stretching region revealed two new peaks upon introduction of the ketone that were attributed to the ketone-bound urea. An observed phase change in these peaks supported that ketone binding not only disrupted the hydrogen-bonded network within the ureido monolayer but also led to significant reorientation of the ureido groups.
Li, Zhiguo, Champika N. Weeraman, Julianne M. Gibbs-Davis
J. Phys. Chem. C